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Disease Alert

A new strain of late blight has been identified in tomato fields in New York State. Here’s why potato growers on both sides of the border would be wise to take notice.

Attending winter trade shows and meetings are a favoured ritual for many potato growers. Certain topics tend to dominate the conference circuit every year, and one that’s likely to have a lot of farmers talking this winter is a brand-new strain of late blight called US-25.

“This is a topic that will be discussed at winter meetings,” says Eugenia Banks, a potato specialist with the Ontario Potato Board. “There’s not much awareness about US-25 yet, and it’s very important to communicate this finding to growers.”

Discovered in three commercial tomato fields in upstate New York State in the summer of 2018, US-25 was found to be a novel strain by plant pathologists at Cornell University, an Ivy League research university in Ithaca, New York. So far, it hasn’t been reported anywhere else in the United States or Canada.

Eugenia Banks is a potato specialist with the Ontario Potato Board. Photo courtesy of Vanessa Currie, University of Guelph.

“It was first identified on June 18,” says Christine Smart, a professor of vegetable pathology at Cornell. “We did the standard genotyping … and it was determined to be different than any genotype we had ever seen before.”

Smart says the origins of this new strain remain a mystery. Her team is planning to meet with officials from the New York State Department of Agriculture and Markets to try to figure out where US-25 came from.

Smart says although US-25 has been found only in tomatoes thus far, Cornell researchers have shown it can infect potato plants under laboratory conditions. For this reason, she’s been keeping potato extension and research pathologists on both sides of border informed of the latest developments around US-25.

Smart says there’s a chance US-25 could jump species and grow to become a dominant late blight strain like US-23 currently is in potato. US-23 infects both tomatoes and potatoes and is very aggressive on potato tubers.

“It’s always possible. Over the years, there’ll be one predominant strain and then that gets overtaken by the next one that comes along. We’ll have to wait and see if that happens with US-25,” Smart says.

“We know that it can survive on potato plants, so there’s no reason to believe that it can’t move to potato. In the field, we just haven’t seen it.”

Smart says there’s also a chance this new strain could simply disappear.

“In the past, we’ve had isolates that were only seen for one or perhaps two seasons, and then not again,” she says. “But we are learning as much as we can about this particular strain because if it does show up again, we want to be able to give growers the absolute best recommendations to help them mitigate losses.”

Smart says the fact that US-25 hasn’t turned up in potatoes yet may suggest it may not be as aggressive as other late blight genotypes like US-23 and US-8, which have afflicted potato crops in recent years.

As Smart points out, “hot and dry conditions just don’t support late blight” so whether US-25 manages to gain a foothold in tomatoes and then possibly potatoes will depend a lot on the weather.

Christine Smart, professor of vegetable pathology at Cornell University. Photo courtesy of Cornell University.

“We had a very dry summer until mid-July and the environment plays a huge role, so we’ll be watching really closely next year to see if we pick up US-25 anywhere else,” she says.

Banks believes if US-25 ends up becoming a dominant species south of the border, there’s little doubt it would eventually turn up in Canada since late blight spores can travel hundreds of kilometres on storm fronts.

Banks says one significant aspect of US-25 is that it’s an A2 mating type, unlike the current predominant strain of late blight, US-23, which is an A1 mating type.

According to Banks, if US-25 infects a tomato or potato plant that’s also infected with US-23, they can mate with each other and produce sexual spores called oospores, which can result in new strains of late blight.

Another problem, she says, is that oospores can remain viable in the soil for years at a time.

As Banks points out, all late blight strains can overwinter in seed tubers, cull piles or tubers left in the field that do not freeze in the winter —but only oospores produced by A1 and A2 mating are capable of overwintering in the soil itself.

“If a field is infested with oospores, the late blight threat will be present in that field from planting to harvest for many years,” says Banks.

Smart agrees that having US-23 and US-25 together would be bad news for potato growers.

“What we don’t want is for both of these isolates to end up in the same field and run the risk of sexual reproduction that then produces these long-lived overwintering spores,” she says, adding that late blight oospores are currently found in some soils in Mexico but not in Canada or the United States.

Smart says in instances where oospores are present in the field, tomato and potato plants will be at risk for late blight infection sooner than normal. In this scenario, she adds, it would make sense for growers to modify their late blight management strategies and start spraying fungicides earlier in the season.

Smart maintains the best way for growers to protect themselves from new late blight strains like US-25 is to keep on doing what they already do. “All the things they do to prevent late blight in any given season, such as good crop rotations and cultural practices like getting rid of cull piles, are going to be the exact same strategies they’d want to use to make sure US-25 doesn’t go anywhere,” she says.

One tool that might not be available in the event that US-25 ever becomes a problem for potato growers is metalaxyl, the active ingredient in Ridomil. The popular fungicide has provided good control for US-23 but US-25 has been shown to be metalaxyl resistant.

“Ridomil is an extremely effective fungicide and it’s really nice to use Ridomil because of its systemic nature, but there are other effective fungicides on the market as well,” says Smart. “I think that taking Ridomil out of the mix to control US-25 with the tools that are available right now might not have a huge impact.”

Banks agrees. “US-25 is resistant to metalaxyl but fortunately there are other reliable late blight specific fungicides that when applied timely and in combination with many cultural practices, can reduce the incidence of late blight,” she says.

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